Observing Sun-Earth Connections

by Dr. Daniel N. Baker
Univ. of Colorado, Boulder

The fleet of spacecraft which compose the ISTP program. Click
here for a larger image.
Image courtesy the
ISTP
program.

Humankind has been fascinated since the dawn of recorded history with
the Sun and its relationship to Earth. Virtually all civilizations have
speculated about the place of our planet in the realm of the solar system
and the stars of our galaxy. Only within the past few hundred years - since
Copernicus - has there been acceptance of Earth revolving around the Sun
and only since the time of Galileo has the changing face of the Sun been
known. The role of the Sun in driving disturbances in Earth's magnetic
field has come to be appreciated within the last century or so.

Thus, human understanding of solar-terrestrial relationships, though
having a history of perhaps five thousand years or more, has proceeded at a
painfully slow pace. Just since the beginning of the space age have we
come to a relatively clear picture of the nature of solar variability and
how this variability affects Earth. Astronomical observations make clear
that our Sun is like many variable stars and so our Sun-Earth system is the
physical prototype for stellar systems throughout the cosmos.

Rather remarkably, human technology on Earth has developed in close step
with our evolving appreciation of the Sun's influences. Mankind is now
using a web of electrical and communication links that literally gird
Earth. We also employ a vast array of spacecraft around Earth that give us
nearly instantaneous communication, exact position information, knowledge
of approaching weather systems, and military intelligence that makes the
world safer for all of us. Every one of these human technologies can be
adversely affected by disturbances in the solar-terrestrial environment.
P>

We now know in general terms that our Sun reaches a maximum of activity
every 11 years. As it reaches this state of coronal disturbance, the Sun is
capable of expelling huge "plasmoids" of material (called coronal mass
ejections - CMEs) which can move outward from the Sun at more than 1000 km/s.
The shock waves preceding such plasma structures can accelerate
particles to huge energies sometimes over one hundred million electron
volts. If the shock waves and CMEs strike Earth's magnetosphere, they can
initiate huge geomagnetic storms that can thoroughly disrupt power systems,
communication links, and the constellations of operational spacecraft on
which our societies increasingly rely. The appreciation of CMEs as the
agents of such profound solar disturbances of Earth and its environs has
only come about in the past few years: This "paradigm" shift has had a far-
reaching impact on how we think about solar-terrestrial relationships.

Given a many thousand-year wait to have the tools to study the Sun,
Earth, and our place in between, we now have a most remarkable situation
presented to us. The International Solar-Terrestrial Physics (ISTP) program
has put into place the most astounding array of spacecraft and ground
facilities ever conceived of for studying the space environment. There are
exquisitely sensitive telescopes in space examining the Sun's many layers.
There are spacecraft measuring the hot, high-speed plasmas flowing over
Earth from the expanding solar corona. There is an unprecedented armada of
spacecraft within Earth's magnetosphere examining continuously all facets
of the plasmas which ebb and flow as the Sun buffets our geospace
environment. There is even an international web of ground stations that are
recording quite exactingly the magnetospheric and ionospheric signatures of
the ever-changing interaction of the variable Sun with our terrestrial
environment. Humankind has never before had a "Great Observatory" of such
power, precision, and completeness to study our most important star - the
Sun - and our most important planet - Earth.

Perhaps ironically - certainly fortuitously - the tools offered to us by
ISTP have fallen into place just as we are beginning a new solar cycle -
Number 23 in the international parlance. As measured by Sunspot number,
this next solar maximum - probably to be reached in the year 2001 - will
most likely be a large one. It may be the equal of the strongest solar
maximum that has been experienced in the modern era. This would mean that
solar disturbances of great power and destructive potential may be on their
way to the geospace domain. It is an historic confluence of immense
importance that the ISTP armada is in place now and is operating flawlessly
as the "Solar Maximum 2001" approaches. We have a possibility - perhaps
never to be repeated - to study all aspects of the solar maximum and its
consequent effects on the near-Earth environment. It is an epochal
occurrence that for modest costs, the extended operation of ISTP and
affiliated spacecraft can give us the scientific view and the practical
knowledge that we need to finally understand the disturbed Sun and the
consequently-disrupted geospace environment. The extended ISTP program
provides us with an opportunity to revolutionize our
understanding of solar-terrestrial physical processes.